1. Field of the Invention
The present invention relates to a multi-pipe once-through type boiler having a small volume, and used, for example, in a domestic heating device.
2. Description of the Related Art
Known in the prior art is an multi-pipe once-through type boiler provided with an inner row of circumferentially arranged pipes, an outer row of circumferentially arranged pipes, a combustion chamber inside the inner pipes, and a tubular combustion gas passageway. The inner row of pipes is provided with an inlet opening to the combustion chamber, and the outer row of pipes is provided with an outlet for connecting the tubular combustion gas passageway with a flue pipe.
The most closely related prior art is Japanese Unexamined Patent Publication (Kokai) No. 58-88502 published on May, 1983. In this prior art, the inner and outer pipes are provided with a plurality of fins to increase the efficiency, of the heat transfer from the combustion gas to water in the pipes.
Other related prior arts belonging the same applicant are Japanese Examined Utility Model Publication No. 59-41361, Japanese Unexamined Patent Publication No. 57-29000, and Japanese Unexamined Patent Publication No. 58-11303. Further related arts other than those of the applicant are; Japanese Examined Utility Model Publication No. 56-54401, Japanese Unexamined Utility Model Publication No. 52-133801, Japanese Examined Patent Publication No. 44-9161, German Patent Publication No. 2248223, and Austrian Patent Specification No. 308771. However, in view of the present demand for energy and cost saving devices, the heat transfer efficiency of the above prior arts is not satisfactory and should be improved.